Sarcoplasmic reticulum Ca2+ regulatory protein gene expression in human right atrium under hemodynamic overload.

Sarcoplasmic reticulum (SR) Ca2+-adenosine triphosphatase (ATPase) mRNA expression is reduced in the failing human myocardium. However, it is not known whether SR Ca2+-regulatory protein gene expression is altered in human myocardial tissue subjected to pressure overload or volume overload. We sought to determine whether SR Ca2+-regulatory protein gene expression is altered in human atrial tissue subjected to mechanical overload. We obtained right atrial myocardial tissue (about 250mg) at open-heart surgery from three groups of patients: no hemodynamic overload to the right atrium (control group; 12 patients), atrial septal defect (ASD group; 8 patients), and tricuspid regurgitation (TR group; 7 patients). We measured the myocyte size, the area of interstitial fibrosis, SR Ca2+,-ATPase, and ryanodine receptor mRNA abundance. The isolated cardiocyte area and the percent area of interstitial fibrosis were in the order TR > ASD > control (P < 0.05). The SR Ca2+-ATPase mRNA level in TR was significantly decreased (P = 0.004) compared with the control, whereas in the ASD group it did not differ significantly from control. There were no significant differences in ryanodine receptor mRNA levels among the three groups. SR Ca2+-ATPase gene expression was downregulated in human atrial tissue with TR but not in ASD, which might have resulted from the differences in the degree and/or the type of hemodynamic overload to the myocardium.

Voltage- and time-dependent block of I(f) by Sr2+ in rabbit sino-atrial node cells.

The hyperpolarization-activated cation current (I(f)) was recorded in single myocytes dissociated from the rabbit sino-atrial node and the Sr(2+)-mediated block of I(f) examined. In the presence of 0.1-10 mM Sr2+, the activation phase of I(f) was followed by a slower decay during hyperpolarization. In the steady state I/V diagram, the Sr2+ block progressed with increasing hyperpolarization. Ba2+ also blocked I(f), but no time dependency could be observed. The blocking effect of Ca2+ was weak, and Mg2+ had little effect on I(f). The relationship between extracellular Sr2+ concentration [Sr2+]o and the block was described by a Hill coefficient of 1. The half-saturating [Sr2+]o were 2.0, 1.6, 1.1 and 0.65 mM at -90, -100, -110 and -120 mV, respectively. The rapid application of Sr2+ during the full activation of I(f) using the jet-stream method induced an exponential decline of I(f). The reciprocal time constants were linearly correlated to [Sr2+]o, suggesting 1:1 binding stoichiometry. The fractional electrical distance for the binding site was approximately 0.5 from the external side of the channel. Based on the multiple closed and open states for the I(f) channel, a mathematical model for the Sr2+ block was constructed in which the time course of I(f) in the presence of Sr2+ was described by the sum of three exponential functions. Fitting the model to the original traces revealed blocking and unblocking rates similar to those obtained from the jet-stream method. At -110 mV, the blocking rate was 410 M-1 s-1 while the unblocking rate was 0.16 s-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Cation-dependent gating of the hyperpolarization-activated cation current in the rabbit sino-atrial node cells.

1. The gating properties of the hyperpolarization-activated cation current (I(f) or Ih) were investigated in single pacemaker cells dissociated from the rabbit sino-atrial node. 2. The whole-cell I(f) was recorded in the presence of different external cations. The inward I(f) was increased when external Na+ was replaced with K+, and was decreased in Li+ or Rb+ solution. In Tris+ and Cs+ solutions, the inward I(f) was negligible. The outward tail current recorded upon depolarization was largest in Li+ solution and smaller in a sequence of Na+, Tris+ and K+ solutions. In Rb+ and Cs+ solutions, only a small tail current was recorded. 3. The outward tail current had a 'shoulder' in Na+ solution, which was much delayed by replacing Na+ with Li+. In K+ solution, the decay of the tail current was much faster, and no obvious shoulder was recorded. The tail current was slowest in Li(+)-rich and 0 mM K+ solution, and was progressively accelerated by adding K+ over the range from 0 to 3 mM. The tail current at 30 mM [K+]o showed only a small shoulder. A common binding site to modulate the I(f) deactivation was suggested for monovalent cations. 4. The shoulder of the I(f) tail became more evident as I(f) was activated to a larger extent either by prolonging the duration or by increasing the amplitude of the preceding hyperpolarization in both Na+ and Li+ solutions. 5. The I(f) was first activated by hyperpolarizing the membrane to -110 mV, and then deactivated by depolarization. The inward tail current at -50 mV showed a single exponential decay. At more positive potentials, the shoulder of the outward tail currents became more evident and the rate of the final decay was increased. 6. The time course of I(f) activation was well fitted with the sum of two exponential functions. Time constants of both components were not affected by the external cation (Na+, K+ or Li+) replacement. Likewise, the quasi-steady state activation was conserved when external Na+ was replaced with Li+. 7. Two closed and three open states were assumed in a sequential state model of the I(f) channel. The cation effects were well simulated by assuming that the deactivation rate was selectively modulated. The flow of I(f) during the spontaneous action potential was calculated. The activation of I(f) started on repolarization to the maximum diastolic potential and reached a maximum in the middle of the diastolic period. Its peak amplitude was 14% of the net inward current during the diastolic period.

External K+ increases Na+ conductance of the hyperpolarization-activated current in rabbit cardiac pacemaker cells.

The hyperpolarization-activated current (I(f)) was recorded from single myocytes dissociated from rabbit sinoatrial node. Although I(f) is usually carried by both Na+ and K+, removal of the minor K+ component from physiological saline suppresses inward component. This inward Na+ current through I(f) channel increases on raising the extracellular K+ concentration. The Na+ conductance relative to K+ conductance (PNa/PK), as measured from the reversal potential, increases and saturates near 5 mM K+. This effect is different from the current increase caused by raising the concentration of carrier ion K+, which saturates at 70 mM with a half-maximal value (K1/2) of 10 mM. It is suggested that the I(f) channel has multiple, interactive binding sites for cation permeation.

Control of the hyperpolarization-activated cation current by external anions in rabbit sino-atrial node cells.

1. Effects of varying concentrations of anions on the hyperpolarization-activated current (I(f)) were studied in myocytes isolated from the rabbit sino-atrial node. Substituting Cs+ for the intracellular K+ clearly separated I(f) from the delayed rectifier K+ current. Control properties, including gating kinetics and ion selectivity, similar to previous studies were obtained. 2. Substitution of extracellular Cl- with larger anions including isethionate, glutamate, acetate, and aspartate, reduced the amplitude of I(f) without changing the reversal potential. Substitution with small anions such as iodide or nitrate supported an intact I(f). These effects were reproduced in the excised outside-out patch conformation. 3. The conductance for I(f) was a saturating function of the extracellular Cl- concentration ([Cl-]o) with an equilibrium binding constant (K1/2) of 11 mM and a slope factor of about 1 when substituted with large anions. Total removal of small anions completely abolished I(f). 4. The voltage-dependent gating of I(f) was not affected by changing ([Cl-]o), suggesting that Cl- modulates conductance properties of I(f). 5. The results indicate that I(f) conductance is unique in that it is dependent on an extracellular anion (Cl-), yet it is carried exclusively by cations, K+ and Na+. These effects are independent of any measurable voltage-dependent gating parameters.

[Surgical treatment of total anomalous pulmonary venous connection under three months of age].

Nine infants under 3 months of age with total anomalous pulmonary venous connection underwent total correction between June, 1986 and September, 1988. The age at operation ranged from 4 days to 59 days, averaging 19 days, and the body weight ranged from 1,814 g to 4,105 g, with a mean of 3,050 g. The types of TAPVC were Darling Ia in 4, Ib in 1 and III in 4. All the patients were operated by the posterior approach under cardiopulmonary bypass with high flow (130-200 ml/kg/min) and mild hypothermia using modified GIK-cardioplegia, topical cooling and aortic cross clamping. Although the incision in the common pulmonary vein trunk was never extended into the pulmonary veins or vertical vein, the length of the mouth was at least 10 mm. In the postoperative management, care was taken to avoid overhydration and rapid volume infusion. Blood pressure was kept just enough to maintain urine output. Heart rate was kept over 170/min for early postoperative days with the use of isoproterenol or atrial pacing. There was no operative or late death. Postoperative course was uneventful in all cases except one with low output syndrome in which mechanical ventilation for 8 days was required. Postoperative catheterization and angiography revealed normal intracardiac pressure values and no pulmonary venous obstruction in all cases. Follow-up period ranged from 10 to 37 months, and there has been no patient with the signs of PVO.